When landing an aerial vehicle, such as an unmanned helicopter, it is important that the aerial vehicle has access to accurate information regarding movement, position and attitude relative to the surface of the landing area. In particular, when landing on the platform of a ship, the sea continuously effects the attitude of the ship and hence the landing platform. The movement of the platform has three degrees of freedom. The inclination is determined by the pitch and the roll, and there is also a movement in the vertical direction, for example if there is a heavy sea, the amplitude of the waves can change the distance between the ship and the vehicle by quite a large amount. It is therefore important to be able to determine when the helicopter is sufficiently parallel to the platform.
U.S. Pat. No. 4,868,567 discloses a landing approach aid for a helicopter provided with a spotlight, a television camera and displaying device, and where the landing field exhibits mirrors. The landing can then be carried out on the basis of the light beams reflected by the mirrors which specify the location of the mirrors on the television image.
U.S. Pat. No. 4,995,722 discloses a helicopter that incorporates means for emitting a conical light beam, and a landing platform on a ship that incorporates photosensitive cells that detect the conical light beam and calculates the attitude of the helicopter. From this landing data are displayed on a panel which the pilot can see in order to land.
U.S. Pat. No. 3,721,499 discloses a night navigation and landing device, wherein a television camera is carried by a helicopter. A video monitor is also provided with scales on which the camera shows the images of two light beams projected from the aircraft. The light beams are linear and parallel and orientated in the observation axis of the camera. When the pilot has reached the zone where he has to land, he locates his landing ground by means of the camera, governs the helicopter by the aid of reflected beams from spotlights on the helicopter and takes then into account his position and the line of descent suitable for the landing, which he has to carry out.
EP 1 555 545 A1 discloses a multi-beam rangefinder system comprising a transmitter arrangement in which pulses are produced sequentially in three fixed non-coplanar directions, and the points which the pulses arrive at the ground define a plane. A planar model passing through these points then approximates the ground whereby it is possible to estimate range derived values including an altitude of the airborne platform relative to the ground, a time to land on the ground, roll and pitch as well as the angular velocity and angular acceleration.
A problem with the systems according to U.S. Pat. No. 4,995,722 and U.S. Pat. No. 4,868,567 are that they are dependent on devices placed on the landing platform, such as photosensitive cells or mirrors. In U.S. Pat. No. 3,721,499 a pilot governs the helicopter by the aid of reflected beams from the spotlights on the helicopter, and in EP 1 555 545 A1 a planar estimation model of the ground is needed to calculate for example a time to land on the ground.
There is thus a need of a landing system which can manage to facilitate autonomous landing independent of devices placed on the landing platform or even a pilot, or as a supplement for or a complement to a complex plane calculation in order to predict a time to land on the ground. There is also a need of determining when the landing has actually been performed.